Design for Test Guidelines and Considerations

Major Benefits of IEEE 1149.7

While the electronics industry continues to produce the next
generation of integrated circuits to meet the ever growing
consumer demand for increased performance, reduced power
consumption and lower cost, new industry-wide challenges are
arising that address how to debug and test these highly
complex devices. Each successive new IC generation
integrates more logic and functionality into a smaller
physical size. It is becoming increasingly more difficult to
balance the practical needs of accessing the internal test
logic and debug circuitry while still meeting the strict
functional performance and cost requirements.

IEEE Standard 1149.1, commonly referred to as JTAG (Joint
Test Action Group), provides a convenient and standardized
method to communicate with embedded devices. The IEEE 1149.1
standard is widely used in boundary-scan testing tools to
detect structural manufacturing defects in the solder
interconnects that may occur during board assembly and as a
platform for developing and debugging embedded software and
firmware during system bring-up and board verification.

While JTAG has been in use for over 20 years, and will
probably continue to be used far into the foreseeable
future, the recently released standard IEEE 1149.7 (Standard
for Reduced-pin and Enhanced-functionality Test Access Port
and Boundary Scan Architecture) has been created to improve
upon it and extend its capabilities. The goal was not to
replace IEEE standard 1149.1 but to create a complementary
standard that addresses the recent changes in the integrated
circuit technology and topology. The new standard builds on
the existing one in order to implement additional
functionality and maximize debug performance while
simultaneously maintaining backwards compatibility.
The new IEEE 1149.7 standard (cJTAG or Compact JTAG)
provides several major benefits to both board designers and
embedded engineers.

Summary of Major New Features

Reduced pin count

Star topology

Individual device addressing

Chip level bypass

Additional power management features

Summary of
Major Benefits

Simplified connections between devices

Improved support for devices with multiple cores

Increased debug performance

Reduced Pin Count

One of cJTAG's most significant extensions to the existing
IEEE standard 1149.1 is the addition of a 2-pin TAP
interface capability where data in transferred using only
the TMS and TCK pins. The TDI and TDO signals may be removed
while still allowing access to the previous functionality or
the new advanced scan protocols. This is significant because
most modern embedded systems integrate multiple ICs and
often have severe size constraints. Dedicated pins for
bringing out chip debug and test functionality is desirable
during board development and production but raises the
overall cost of the devices. It is costly both in terms of
money and real estate to dedicate special pins for bringing
debug and test signals out, especially as device vendors are
pressured to make the IC packages smaller.

By only requiring a two-pin interface for operation instead
of the four pins required in IEEE 1149.1, the 1149.7
standard allows more test and debug functionality to reside
on a reduced number of external pins. Fewer pins, nets and
discrete components like pull-up resistors also makes
routing and trace layout much easier, particularly in
applications like stacked-die devices and multi-chip modules
where various components are vertically stacked. Reducing
the number of pins needed for test also lowers the overall
packaging cost, frees up pins for implementing additional
features and functionality and helps device and board
designers meet their form factor and cost constraints.

Star Topology

The star topology is one of several enhancements in the IEEE
1149.7 standard for gracefully handling arrays of identical
devices and devices with multiple cores. These can be boards
with 32 DSPs or multi-core CPU or Systems-on-Chip (SoC) with
separate physical processors, stacked die configurations or
multichip System-in-Package (SiP) modules with several
distinct peripherals within the same physical package.
The introduction of a star topology in the IEEE 1149.7
standard complements the reduced pin count. Designers
working with stacked-die devices, multi-chip modules and
plug-in cards will favor the star topology and 2-pin
interface because it simplifies the physical connections
between devices.

Chip Level Bypass and Individual Device Addressing

Not only does the IEEE 1149.7 standard better support
devices with multiple cores and internal peripherals, it
also does so more efficiently. The serial design
architecture of the existing JTAG standard made it very
difficult to communicate exclusively with one specific
device in the scan chain due to interactions with other
devices in the chain, particularly when multiple devices or
cores are combined into one physical package. The standard
provides a method to address and access specific devices in
the scan chain individually, without having to shift bits
through the entire instruction register length of the full
scan chain.

A chip level bypass mechanism has been implemented to reduce
the overall scan chain length by putting unused devices in a
1-bit chip bypass mode. Utilizing this feature can make very
long scan chains dramatically shorter and improve the
overall scan efficiency and throughput.

The individual addressing and chip level bypass capability
provided by the new standard allows the host controller to
communicate with only the intended device. This allows for
increased performance and extends the capability of the
current standard by enabling more advanced debug and
instrumentation logic to be designed into the individual
chips. The new standard provides a common method to
communicate with the enhanced functionality and the host can
now address multiple internal modules through the same 2 or
4-pin TAP interface.

The ability to quickly access a specific device in a system
with multiple devices also directly improves the ease of
debugging a complicated system. This is extremely important
because the International Technology Roadmap for
Semiconductors (ITRS) currently expects that the number of
internal cores will roughly double with each new processor
generation.

Power Management

One limitation of the IEEE 1149.1 (JTAG) standard is that
devices only have a single power state while being tested. A
device has to be either completely on or completely off
while the boundary-scan tests are executing.

The IEEE 1149.7 standard provides a standardized interface
with four selectable power modes to control the device
power. This capability to control the power consumption of
the debug logic or individually adjust the power state of
multiple cores in a device will make boundary-scan testing
much easier in many board designs.